In the plating of dielectric (non-conductor) substrates by chemical (electroless) plating, it is well known that suitable catalytic pretreatment is a prerequisite for effective electroless metal deposition. Such practices are well known and accepted in the art.
In examining the prior art for catalytic pretreatment it appears that while different procedures have been used, the incorporation of precious metals (e.g. palladium containing solutions) was common to all procedures. One catalytic system of particular interest is the two step process as disclosed in U.S. Pat. No. 3,011,920. The effective catalyst is proposed to be a colloid of an elemental precious metal (e.g. palladium) stabilized by the excess stannous chloride present in the medium. While the system disclosed in U.S. Pat. No. 3,011,920 has been quite popular in commercial practices, rising costs of precious metals, the high acidic content, and miscellaneous product reliability problems have led to the quest for new systems in which the use of precious metals, tin, as well as hydrochloric acid could be completely eliminated.
In meeting this objective it was found, as described in U.S. Pat. No. 3,993,799, that colloidal systems based upon non-precious metals could constitute the basis for catalysts in new commercial plating processes. More specifically it was found that colloid metals, alloys and hydrous-oxides of non-precious metals (preferably of the group consisting of metals of Period 4 and groups IB and VIII of the Period Table of the Elements) may be used in the direct replacement of palladium catalysts used in the art followed by a treatment in a suitable activating medium (which is optional); and thereby providing more energetic catalytic sites capable of initiation of the electroless process. In addition, colloidal matter of compounds bearing the catalytic metals are also of potential use in the present process.
In another system (U.S. Pat. No. 3,993,491) it was found that copper ions in excess of stannous ions may also constitute the basis for the catalysis of non-conductors. Still another system utilizing non-precious metal ions is described in U.S. Pat. Nos. 3,772,056 and 3,772,078 issued to Polichette et al. In the systems described therein, the dielectric substrate to be plated is immersed in a solution of nonprecious metal ions, and then dried to promote adherence of the ions to the dielectric surface. The retained ions may then be reduced to the metallic state by heat, light or chemical means. A system of this nature is a significant departure from the standard commercial process as a result of the required intermediate drying step to achieve the required adhesion. In standard commercial processes, the article to be plated is passed from one aqueous treating tank to another with intermediate rinsing steps to remove excessive material and hence minimize cross-contamination. A procedure of this nature is essential due to the automation of most plating operations and thus the Polichette et al procedure would require a major modification in standard equipment and procedures, thus rendering the process unattractive and costly. A process for electroless plating, to be commercially feasible, should be adaptable to existing commercial processing techniques.
The present invention and procedures are intended to improve the performance of such non-noble (preferably consisting of the metals of Period 4, and Groups IB and VIII of the Periodic Table of the Elements) based catalytic systems. However, it is not to be limited to any specific manner or mechanism by which these metals are applied (true solutions vs. colloidal dispersions or emulsions) to or found on the surface (i.e., oxidation state) of the substrate to be plated.